The relative stabilities of the H2GeX,
trans-HGeXH, and cis-HGeXH (X = O, S, Se)
species and the transition
states for H2GeSe ↔ trans-HGeSeH and
trans-HGeSeH ↔ cis-HGeSeH isomerizations were
investigated
using post-Hartree−Fock ab initio methods. Geometry optimization
and frequency calculations were performed
at the HF, MP2, DFT, and QCISD(T) levels using
TZP(2d,2p), TZP(2df,2pd), and
TZP++(2df,2pd) basis
sets. In the cases of oxygen and sulfur, the isomers
H2GeO and H2GeS represent the
structures with the
highest energy, and the global minima corresponds to the
trans-HGeSH and cis-HGeOH forms,
respectively.
In a more detailed study of the potential energy surface of the
Ge[H2Se] system, we have found that
the
trans-HGeSeH structure is a global minimum separated at the
QCISDT/TZP(2df,2pd) level by only 0.5 and
2.0 kcal/mol from H2GeSe and cis-HGSeH,
respectively. In all cases the electron correlation energy
plays
a dominant role, and reliable assignment of the relative stability of
these energetically close-lying isomers
was possible using only higher levels of ab initio theory.
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